Early stages of Y chromosome degeneration revealed

In a first, biologists have found a way to shed light on early stages of Y-chromosome degeneration - thus rewriting the evolutionary history of sex chromosomes - in plants.

Till date, it was now well understood what happens to the Y chromosome during the earliest stages of this evolution, or the time scales over which degeneration occurs.

In humans, the Y chromosome has undergone extensive gene loss over its roughly 200-million-year evolutionary history and now retains only about three percent of its ancestral genes.

The most well-studied Y chromosomes, including those in humans and other animal species, began degenerating hundreds of millions of years ago. Not so with plants.

"The emergence of separate sexes in plants is a relatively recent evolutionary innovation, making them ideal for this study. Only about six percent of flowering plants have males and females. The remainder are hermaphrodites," explained professor Spencer Barrett from University of Toronto (U of T).

The scientists used a plant species with separate sexes whose X and Y chromosomes probably first evolved around 15 million years ago at the most, making them relatively young compared to those in animals.

"We tested for Y-chromosome degeneration in Rumex hastatulus, an annual plant. We found that genes on the Y chromosomes have already started to undergo genetic degeneration, despite their relatively recent origin," said Josh Hough from U of T's department of ecology and evolutionary biology.

"The sex chromosomes in Rumex hastatulus are particularly interesting because of the recent emergence of a new sex chromosome system, in which some males carry a second, even younger, Y chromosome," Hough added.

"This gave us a key time point to understand the chronology of Y-chromosome evolution. Remarkably, even these genes were already showing early signs of degeneration," said professor Stephen Wright from U of T.

The paper was published in the journal Proceedings of the National Academy of Sciences.